Because of high quality, energy saving, thin body and wide application of the advantages, OLED liquid crystal display device is widely used in mobile phones, personal digital assistants, digital cameras, laptops, desktop computers, television and other consumer electronics products, playing the mainstream of the display device.
As shown in FIG. 1, which is a common pixel driving circuit to OLEDs. Due to the transistor T2 operating in a long period of time, the threshold voltage Vth drifts in the saturation region, and resulting in a luminance unevenness (mura) for the panel display screen. To improve the brightness unevenness, the PWM (digital drive) mode is presented. Sub-field cutting method is the commonly used PWM drive mode. The simplest structure is 3T1C pixel circuit, as shown in FIG. 2, Data is the data drive signal, Gate1 is the charging scan signal, the control transistor T1 charges the point A, Gate2 is the discharge scan signal, the control transistor T3 discharges the point A, Vini is the reference voltage, which is around zero. A outputs only two Gamma voltages, namely GM1 (brightest) and GM9 (darkest) two voltage levels. According to the transistor current voltage I-V equation:Ids,sat=k*(Vgs−Vth,T2)2=k*(VA−VB−Vth,T2)2 
Among them, Ids,sat is the transistor conduction current, k is the intrinsic conductivity factor, Vgs is the transistor gate voltage, Vth, T2 is the threshold voltage of the transistor T2, VA is the voltage of point A. VB is the voltage of point B. Due to the degradation or non-uniformity of the device, the variation ΔVth of the transistor Vth changes little relative to (VA−VB). Therefore, relative to the analog voltage drive mode, PWM drive can inhibit the OLED brightness uneven problem.
However, when the image is grayscale 255, the mura of the panel has been very serious, PWM driving method cannot eliminate the mura under grayscale 255, resulting in the actual display of the LCD panel is not ideal. As shown in FIG. 3, FIG. 3 shows a 8 bits sub-field and other cut PWM drive diagram. The x-axis represents time, and the y-axis represents scanning time of the scan line. A frame is cut into 8 sub-fields with the same time, by controlling the charge and discharge time period of PWM driving circuit to achieve the pixels has different lighting time in different sub-fields, the pixel lighting time duty ratio is 1/128, 1/64, 1/32, 1/16, ⅛, ¼, ½, 1, so that the grayscale of the display panel is different. When the grayscale is 255, the panel brightness uneven phenomenon has been very serious, and PWM driving circuit sub-field cutting drive method cannot eliminate the panel 255 grayscale brightness uneven phenomenon, grayscale 255 pixels show the brightness of the uneven phenomenon will directly affect the display effect to other grayscale pixels, resulting in the actual display effect of the display panel is not ideal.